1. Field of the Invention
The present invention relates to a thin film magnetic head, and particularly relates to a device structure of the thin film magnetic head comprising a pair of magnetic layers where a direction of magnetization is changed relative to an external magnetic field.
2. Description of the Related Art
Associated with high recording density of a hard disk drive (HDD), a supersensitive and high-power head is in demand. As a head fulfilling this request, a spin-valve head has been invented. A pair of ferromagnetic layers via a nonmagnetic middle layer are established in this spin-valve head. An antiferromagnetic layer is arranged by making contact with one of the ferromagnetic layers, and the direction of magnetization of the ferromagnetic layer is fixed to one direction due to an exchange coupling with the antiferromagnetic layer. In the other ferromagnetic layer, its direction of magnetization freely rotates relative to the external magnetic field. This ferromagnetic layer is also referred to as a free layer. In the spin-valve head, a change in magnetoresistance is realized by a change in a relative angle of spins in these two ferromagnetic layers. The pair of ferromagnetic layers are interposed by a pair of shield layers, and an external magnetic field from an adjacent bit on the same track of a recording medium is blocked.
The exchange coupling between the antiferromagnetic layer and the ferromagnetic layer is one of the essential characteristics in the spin-valve head. However, in view of further high recording density advances, and when the read gap (width in a traveling direction of a medium signal when the medium signal is read by a magnetic head; correlated to a thickness of a film interposed in between shields) becomes approximately 20 nm, there is no space to contain the antiferromagnetic layer within the read gap. Then, a technology to control the direction of magnetization of the ferromagnetic layer and to change a relative angle formed with the directions of magnetizations of two ferromagnetic layers according the external magnetic field in some way is required. A thin film magnetic head having two free layers whose directions of magnetization change according to the external magnetic field and a nonmagnetic middle layer interposed by these free layers is disclosed in the specification of U.S. Pat. No. 7,035,062. The two free layers are exchange-coupled according to RKKY (Rudermann, Kittel, Kasuya and Yoshida) interaction via the nonmagnetic middle layer, and they are magnetized antiparallel to each other in the state where no magnetic field is applied at all (hereafter, this state is referred to as a nonmagnetic field state). A bias magnetic layer is established on rear surfaces of the two free layers and the nonmagnetic middle layer viewed from the air bearing part surface (ABS), and a bias magnetic field is applied in a direction at right angles to the ABS. The directions of magnetization of the two free layers form a constant relative angle due to the magnetic field from the bias magnetic layer. When an external magnetic field in the direction at right angles to the ABS is provided from the recording medium, the directions of magnetization of the two free layers are changed, and as a result, the relative angle formed with the directions of magnetization of the two free layers is changed and electrical resistance of the sense current is changed. It becomes possible to detect the external magnetic field by utilizing this characteristic. As described above, in the film configuration using the two free layers, because the antiferromagnetic layer becomes unnecessary, there is potential where the film configuration is simplified and the reduction of a read gap becomes easy. Furthermore, in this specification, “parallel” means that directions of magnetization are in parallel with each other and both components are orientated toward the same direction, and “antiparallel” means that directions of magnetization are in parallel with each other; however, both components are orientated toward an opposite direction from each other.
However, in the thin film magnetic head with a method where two free layers are magnetically tied due to the RKKY interaction, a material utilizing as a nonmagnetic middle layer is limited and the improvement of a rate of change in magnetoresistance cannot also be expected. For example, Cu shows the RKKY effect and has superior spin conduction; however, because the resistance is too low, it is not the most appropriate as a nonmagnetic middle layer in the film configuration using the two free layers. Then, another technology to magnetize the two free layers to directions antiparallel to each other becomes required.